1. Field of the Invention
The present invention relates to an image capturing apparatus such as a digital still camera or a digital video camera that performs focus adjustment using an image signal obtained by an image sensor.
2. Description of the Related Art
Digital cameras and video cameras often use a so-called contrast detection autofocus (hereinafter, abbreviated as “AF”), which is an AF method for focusing on a subject by detecting, using output signals from an image sensor such as a CCD sensor or a CMOS sensor, a signal depending on the contrast of the subject.
In the contrast detection AF, contrast evaluation values are obtained while changing the focal position of an imaging optical system, and the focal position at which the contrast evaluation value is the local maximum value is detected as an in-focus position. Accordingly, there is the problem that the focus adjustment direction to approach the in-focus position is unclear at the start of focus adjustment in a defocus state. If focus adjustment is started in the state in which the focus adjustment direction is unclear, the focus adjustment is likely to be performed in a wrong direction. In this case, based on a reduction in the contrast evaluation value, the fact that the focus adjustment is being performed in a wrong direction can be detected, but the focus adjustment requires a longer time.
In order to solve such a problem, a method for detecting a focus adjustment direction to approach an in-focus position at the start of focus adjustment, using chromatic aberration caused by an imaging optical system has been proposed (Japanese Patent Laid-Open No. 2009-103800). In Japanese Patent Laid-Open No. 2009-103800, image sensors that respectively receive beams of light that have different wavelengths are provided, and a focus adjustment direction to approximate an in-focus state can be detected based on a difference between outputs of the image sensors. At that time, in order to correspond to the spectral distribution of a subject image, output signals of the wavelengths are normalized with a difference between the maximum and minimum values of luminance values in a focus detection area, and the normalized output signals are used to detect the focus adjustment direction. Accordingly, even if the spectral distribution of the subject image is uneven and the subject has, for example, a reddish color, it is possible to detect the focus adjustment direction by comparing the normalized contrast evaluation values of the wavelengths.
Furthermore, in Japanese Patent Laid-Open No. 2009-103800, reliability determination whether or not the focus adjustment using chromatic aberration can be performed with accuracy is performed, based on a difference between the maximum and minimum values of the luminance values in the focus detection area. Accordingly, if it is considered that the focus adjustment using chromatic aberration is less reliable, normal contrast detection type focus adjustment will be performed, making it possible to ensure the focus adjustment accuracy.
However, depending on the spectral distribution of the subject image, there is a case where it is not possible to correctly detect the focus adjustment direction to approach the in-focus position. For example, if a tone is varied in the focus detection area, namely, for example, a part of the focus detection area is green and another part thereof is red, the focus adjustment direction cannot correctly be detected by the method disclosed in Japanese Patent Laid-Open No. 2009-103800. This is because a difference between the maximum and minimum values of the luminance values in the focus detection area cannot express the spectral distribution in the focus detection area correctly.
Furthermore, for the same reason, there is the risk that reliability determination using a difference between the maximum and minimum values of the luminance values in the focus detection area is made erroneously.